The present invention relates to a reinforcing metallic cord to be used in particular in elastomer-matrix composite articles of manufacture, specifically in pneumatic tires. The cord comprising a plurality of elementary wires twisted together about the longitudinal axis of the cord, which cord, under any drawing condition with a tractive load not exceeding 5 kg, has at least one right cross-section provided with at least one inlet port to enable access of the elastomer material to the inside of the cord section.
The present invention also concerns a process for the manufacture of said cord, said process comprising the steps of: preforming a plurality of elementary wires, submitting them to a permanent bending set along their longitudinal extension; laying the wires together by a double helical twisting about the longitudinal axis of the cord.
The invention further relates to an apparatus for the manufacture of said cord, said apparatus comprising: a bearing structure; an impeller in engagement with the bearing structure and drivable in rotation according to a given axis; a nacelle oscillatably linked to the bearing structure according to an oscillation axis coincident with the rotation axis of the impeller. The apparatus also has feeding means operatively mounted on said nacelle to feed a plurality of wires from respective supply reels, said wires being guided onto the impeller along a laying path having end stretches coincident with the rotation axis of the impeller and a central stretch spaced apart from said rotation axis. At least one preformer is operatively engaged with the nacelle and acting on the wires over a portion thereof upstream of the first end stretch of the laying path.
The above cord is particularly conceived for use in making tire components for motor vehicles, such as the carcass and/or belt plies for example, but it can be easily employed for the manufacture of other articles as well, such as pipes for high-pressure fluids, belts, conveyor belts or any other article of elastomer-based composite material. The metallic cords usually employed as the reinforcing structure for articles of manufacture of elastomer material are generally comprised of a plurality of wires helically twisted about an axis coincident with the longitudinal extension of the cord. Usually, the cords of this type are made by a laying machine of the so-called double-twisting type, provided with an impeller operatively in engagement with a bearing structure and operable in rotation by motor means, as well as a so-called nacelle oscillatably linked to the bearing structure along an axis coincident with the rotation axis of the impeller. The nacelle carries a plurality of supply reels on which the wires have been previously wound, which wires through appropriate feeding and guide means are picked up and guided onto the impeller along a predetermined laying path. This laying path has a first end stretch coincident with the rotation axis of the impeller, a central stretch stepping over the impeller so as to be spaced apart from the rotation axis, and a second end stretch again coincident with the above specified rotation axis.
Rotation of the impeller gives rise during two successive steps and at the end stretches of the laying path, to twisting of the wires and, as a result, formation of the cord, according to a helical winding pitch depending on the relation existing between the rotation speed of the impeller and the pulling speed imposed to the wires upon the action of collecting means usually operating downstream of the laying machine, directly on the cord.
Generally, before being submitted to twisting the wires are subjected to a preforming step by their passage over a preforming device imposing a permanent bending set to the wires themselves, in order to promote the subsequent arrangement of the wires in a helical form to ensure maintenance of the structural compactness of the cord.
The foregoing being stated, in order to eliminate the risk that the cords may undergo undesired corrosion phenomena once they have been introduced into the tire or another article of manufacture of elastomeric material, it is of the greatest importance that the wires forming the cords should be completely coated, over the whole surface extension thereof, with the elastomeric material into which the cord itself is incorporated.
The above result, which is increasingly more difficult to achieve with the increasing of the structural complexity of the cord, cannot be easily attained even when the cords have a low number of wires, which solution, due to the light weight involved, is of particular interest in the production technology of motor vehicle tires.
Said difficulty originates from the fact that, in order to give the cord the necessary geometric and structural stability, the wires are usually intimately compacted in contact with each other, so as to confine one or more closed cavities extending longitudinally within the cord. These cavities clearly cannot be easily reached by the elastomer material during the usual rubberizing steps of the cord.
When for instance, as a result of cuts or punctures caused in the tire structure or for any other reason, humidity and other external agents can penetrate into said cavities, a quick corrosion process of the wires inevitably occurs, to the detriment of the structural resistance of the cord and the tire as a whole.
In an attempt to overcome this problem, so-called "swollen" cords have been proposed, that is, cords in which the wires (generally three to five in number) are maintained always spaced apart from each other during the rubberizing step, carried out by known processes that keep the tractive load applied to the cord to values not exceeding five kilos. An example of these cords is given in the Italian Patent No. 1,099,869 of the assignee.
The result of a complete rubberizing of the wires is thus achieved, but the cords of this type have some use problems in that the wires keep spaced apart from each other also when the cord is submitted to a strong tensile stress during the tire manufacture and when the tire is run, and this condition causes an undesired geometric and structural instability of the cord as a whole, which is substantially prejudicial to the tire behavior.
Alternatively, cords have been proposed which have still a low number of wires, in which at least one strand is deformed so as to acquire a broken line course, such as those described in the U.S. Pat. No. 5,020,312.
In this manner, a continuous contact between at least two adjacent wires along the longitudinal extension of the cord is made impossible, thereby maintaining separation areas between said two wires, that is, ports for admittance of the rubberizing material, at each zig-zag bending of the strand.
A drawback present in this type of cords is a decay in the fatigue resistance values and a consequent decay in the quality of the tire.
Finally, the use of so-called dual diameter cords has been proposed, that is, cords provided with two pairs of wires in which the strand diameter of one pair is suitably differentiated from that of the other pair.
In this connection, publication RD 22 404 points out that such a cord, to be obtained by usual laying machines of the above described double-twisting type causes the important central cavity, which is defined internally of the cords provided with four or five wires of same diameter, to be replaced by two opposite cavities of much more reduced sizes, that can be more easily filled with the elastomer material used for rubberizing.
In spite of this size reduction, said cavities are in any case closed to the outside. This condition makes it difficult to cause the elastomer material to penetrate into the inner parts of the cord section.
Patent EP 0 168 857 discloses a metallic cord for the manufacture of which one pair of wires of the same diameter and a second pair of wires of a smaller diameter than that of the first pair, are fed to a conventional internal collection laying machine, after passing through a circular preforming head where the wires of the first and second pairs follow specific paths to be submitted to preforming in a suitably differentiated manner with respect to each other.
The cord thus achieved has the pair of wires of greater diameter helically twisted together in a mutual contact relationship, whereas the wires of the second pair are each inserted between the two wires of the first pair and extend parallel to the latter, while maintaining suitably spaced apart therefrom.
In this manner, the presence of closed cavities in the transverse section of the cord is eliminated and, as a result, the complete coating of the wires with the elastomer material employed during the rubberizing step is ensured.
However, the wires of smaller diameter keep spaced apart from those of greater diameter also when the cord is subjected to tensile stress under use conditions, which, as in the swollen cords, will cause a certain geometric and structural instability of the cord as a whole, said instability being undesirable.
In addition, it is very difficult to give the cord a precise and regular geometric configuration at each point of its longitudinal extension, in that constancy in the mutual positioning of the wires in the cord is ensured by the particular type of preformer used, but distance of the wires of smaller diameter from those of greater diameter tends to vary randomly at the different points of the longitudinal extension thereof, both under rest conditions and under use conditions of the cord.